LONDON, July 19 (Reuters) - British Prime Minister David Cameron said on Saturday that if it were proven that Ukrainian separatists were behind the downing of a Malaysian jet carrying 298 passengers, Russia would be to blame for having destabilised the country.

"If this is the case then we must be clear what it means: this is a direct result of Russia destabilising a sovereign state, violating its territorial integrity, backing thuggish militias and training and arming them," he wrote in The Sunday Times newspaper.

Cameron also appeared to criticise fellow members of the European Union for being slow to act against the Kremlin.

"For too long there has been a reluctance on the part of too many European countries to face up to the implications of what is happening in eastern Ukraine," he wrote.

"It is time to make our power, influence and resources count. Our economies are strong and growing in strength.

"And yet we sometimes behave as if we need Russia more than Russia needs us." (Reporting by Costas Pitas; Editing by Eric Beech)